Due to the extremely high efficiency afforded to optical design, attempts have been made to manufacture optical elements such as lenses, particularly aspherical lenses, out of high refractive index, high-dispersion optical glass by precision press molding. In precision press molding, reducing the temperature during press molding effectively extends the service life of the pressing mold. Thus, large amounts of lead oxide had been incorporated into conventional glass for precision press molding having a high refractive index and high dispersion to impart a low softening property. For example, Japanese Unexamined Patent Publication (KOKAI) Heisei No. 1-308843 (Reference 1) discloses a glass containing 30-58 weight percent of PbO. Further, Japanese Unexamined Patent Publication (KOKAI) Heisei 7-247135 (Reference 2) discloses a glass containing 25-54 weight percent of PbO. However, precision press molding is normally conducted in an inert atmosphere or weakly reducing atmosphere to prevent oxidation of the pressing mold, particularly the molding surface. During press molding of the glasses of the above-cited references, which contain large lead oxide components, the lead oxide at the surface of the glass is reduced, precipitating as metallic lead on the glass surface. This metallic lead then adheres to the pressing mold, not only compromising the precision of the molding surface of the pressing mold, but also requiring maintenance to remove it. Such maintenance is unsuited to mass production. Further, the melting of the glasses disclosed in References 1 and 2 above containing large amounts of lead oxide poses environmental problems. Thus, the glasses disclosed in References 1 and 2 above are unsuitable for precision press molding.
Among the optical glasses that are currently commercially available, there are optical glasses of high refractive index and high dispersion that have been lightened without incorporating lead oxide, such as the glass disclosed in Japanese Unexamined Patent Publication (KOKAI) Showa No. 62-3103 (Reference 3). However, when these glasses are employed in precision press molding, a high precision press molding temperature of greater than or equal to 650° C. must be employed, resulting in substantial deterioration of the precision pressing mold material and greatly complicating mass production. Such glasses are thus impractical. Further, since the glasses themselves are unstable, crystals tend to precipitate in the glass during precision press molding; even when a mold material capable of withstanding high temperatures is employed, there is a problem in that the yield of precision press-molded products is extremely low. That is, the higher the precision press-molding temperature, the greater the problems with oxidation and deterioration of the mold material, the more difficult it becomes to maintain the precision of the molding surface, and the more difficult it is to mass produce optical elements by precision press molding. Accordingly, there is a need to develop a high refractive index, high-dispersion optical glass for precision press molding with as low a glass transition temperature and sag temperature as possible without incorporating lead.
For example, Japanese Unexamined Patent Publication (KOKAI) Heisei No. 5-51233 (Reference 4) discloses a high refractive index, high-dispersion optical glass the composition of which, denoted as weight percentages, is 10 to 20 percent of SiO2, 3 to 15 percent GeO2, 0 to 7 percent B2O3, where the combined quantity of SiO2, GeO2, and B2O3 is from 20 to 27 percent; 19 to 29 percent TiO2, 17 to 29 percent Nb2O5, 0 to 7 percent BaO, where the combined quantity of Nb2O5, TiO2, and BaO is 44 to 54 percent; 0 to 3 percent Li2O, 7 to 18 percent Na2O, 0 to 22 percent K2O, 0 to 20 percent Cs2O, where the combined quantity of Li2O, Na2O, K2O, and Cs2O is 24 to 33 percent; with a sag temperature of less than or equal to 550° C., a refractive index of greater than or equal to 1.76, and an Abbé number of less than or equal to 26.5. Although this glass achieves the objective of a low sag temperature, the use of a large amount of TiO2 causes coloration of the glass, and there are problems with the stability and melting properties of the glass during mass production. Further, the essential component GeO2 is extremely expensive, precluding the low-cost production of optical glass lenses. Still further, the glass described in Reference 4 has a high liquid phase temperature and a strong tendency to devitrify near the softening point, greatly complicating the production of glass preforms for precision press molding and rendering this glass unsuited to precision press molding.
Ordinary precision press molding is conducted within a temperature range of about 20 to 60° C. higher than the sag temperature of the glass. When the glass sag temperature exceeds 600° C., the pressing temperature becomes 620° C. or more, and OH radicals adhering to the surface of the glass react with the mold material, which ends up decomposing. This decomposition reaction leaves numerous bubbles on the surface of the press-molded glass lens, precluding the maintenance of the precision of the transferred surface of the precision press molded lens and damaging the surface of the molding material, which are clearly unsuited to mass production.
Accordingly, the present invention has for its object to provide an optical glass having a high refractive index and high dispersion that is suited to precision press molding; a preform comprised of this glass that is suited to precision press molding and a method of manufacturing the same; and an optical element comprised of this optical glass and a method of manufacturing the same.